In an LTE (Long Term Evolution) radio communication system that is currently being standardized in relation to the 3GPP (Third Generation Partnership Project), OFDMA (Orthogonal Frequency Division Multiple Access) is applied to the downlink communication from a radio base station to a radio terminal.
In an LTE radio communication system, a two-dimensional scheduler is used in scheduling by assignment of radio resources that are divided by a time direction and a frequency direction to radio terminals so that a radio base station can manage a plurality of radio terminals and make effective use of radio resources. The use of a two-dimensional scheduler by the radio base station enables the assignment of radio resources having different frequencies to respective radio terminals during time-direction scheduling. In this manner, more efficient transmission is enabled in response to frequency variation or time variation in the wireless environment.
When executing scheduling by use of the above two-dimensional scheduler, the radio base station must comprehend the communication quality with the radio terminal. In an LTE radio communication system, the radio base station enables recognition of the downlink communication quality by use of CQI (Channel Quality Indicator) from the radio terminal.
CQI corresponds to MCS (Modulation and Coding Scheme) that is uniquely determined by the modulation scheme and the transmission block size. Further, an LTE radio communication system is specified so that the FER (Frame Error Rate) does not exceed 10%. Therefore, the SINR (Signal to Interference and Noise Power Ratio) that satisfies the condition that the FER is 10% in MCS corresponding to the CQI from the radio terminal may be deemed to be the minimum downlink SINR in the MCS.
SINR that satisfies the condition that the FER is 10% in a predetermined MCS enables advance calculation by use of a computer simulation, or the like. The calculated value may be used as a threshold value when the radio base station selects MCS of a downlink radio resource based on the SINR corresponding to the CQI from a radio terminal.
In an LTE radio communication system, S-CQI (Subband CQI) being the CQI of each subband is used in assigning a frequency-direction radio resource. When a radio base station assigns a plurality of subbands to a predetermined radio terminal, E-SINR (effective SINR) is used to determine the MCS for the radio terminal. E-SINR is a value that combines the SINR corresponding to the S-CQI of each subband assigned to the radio terminal.
A combining scheme of obtaining E-SINR includes a scheme termed EESM (Exponential Effective SIR Mapping) (see Non-Patent Literature 1, for example). In an EESM scheme, the true value of SINR corresponding to S-CQI for each subband assigned to the radio terminal is converted to EESM using an exponential function (Equation 1) EESM=Exp (−SINR/β). As used herein, β is a value termed an EESM coefficient. Further, the value EESM− that is the EESM average value is converted to E-SINR by a logarithmic function (Equation 2) E-SINR=−β*log(EESM−) that is an inverse operation of the above exponential function.
E-SINR is compared with the threshold value for E-SINR that satisfies the condition that FER is 10% to thereby determine MCS that satisfies the condition that FER is 10%.